4,916 research outputs found
Graphene: A Pseudochiral Fermi Liquid
Doped graphene sheets are pseudochiral two-dimensional Fermi liquids with
abnormal electron-electron interaction physics. We address graphene's Fermi
liquid properties quantitatively using a microscopic random-phase-approximation
theory and comment on the importance of using exchange-correlation potentials
based on the properties of a chiral two-dimensional electron gas in
density-functional-theory applications to graphene nanostructures.Comment: 15 pages, 4 figures, submitte
Voltage and temperature dependencies of conductivity in gated graphene
The resistivity of gated graphene is studied taking into account electron and
hole scattering by short- and long-range structural imperfections the
characteristics of disorder were taken from the scanning tunneling microscopy
data and by acoustic phonons. The calculations are based on the quasiclassical
kinetic equation with the normalization condition fixed by surface charge. The
gate-voltage and temperature effects on the resistance peak, which is centered
at the point of intrinsic conductivity, are found to be in agreement with the
transport measurements.Comment: 4 pages, 4 Fig
Chiral tunneling and the Klein paradox in graphene
The so-called Klein paradox - unimpeded penetration of relativistic particles
through high and wide potential barriers - is one of the most exotic and
counterintuitive consequences of quantum electrodynamics (QED). The phenomenon
is discussed in many contexts in particle, nuclear and astro- physics but
direct tests of the Klein paradox using elementary particles have so far proved
impossible. Here we show that the effect can be tested in a conceptually simple
condensed-matter experiment by using electrostatic barriers in single- and
bi-layer graphene. Due to the chiral nature of their quasiparticles, quantum
tunneling in these materials becomes highly anisotropic, qualitatively
different from the case of normal, nonrelativistic electrons. Massless Dirac
fermions in graphene allow a close realization of Klein's gedanken experiment
whereas massive chiral fermions in bilayer graphene offer an interesting
complementary system that elucidates the basic physics involved.Comment: 15 pages, 4 figure
Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides
Motivated by the triumph and limitation of graphene for electronic
applications, atomically thin layers of group VI transition metal
dichalcogenides are attracting extensive interest as a class of graphene-like
semiconductors with a desired band-gap in the visible frequency range. The
monolayers feature a valence band spin splitting with opposite sign in the two
valleys located at corners of 1st Brillouin zone. This spin-valley coupling,
particularly pronounced in tungsten dichalcogenides, can benefit potential
spintronics and valleytronics with the important consequences of spin-valley
interplay and the suppression of spin and valley relaxations. Here we report
the first optical studies of WS2 and WSe2 monolayers and multilayers. The
efficiency of second harmonic generation shows a dramatic even-odd oscillation
with the number of layers, consistent with the presence (absence) of inversion
symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show
the crossover from an indirect band gap semiconductor at mutilayers to a
direct-gap one at monolayers. The PL spectra and first-principle calculations
consistently reveal a spin-valley coupling of 0.4 eV which suppresses
interlayer hopping and manifests as a thickness independent splitting pattern
at valence band edge near K points. This giant spin-valley coupling, together
with the valley dependent physical properties, may lead to rich possibilities
for manipulating spin and valley degrees of freedom in these atomically thin 2D
materials
Universal Dynamic Conductivity and Quantized Visible Opacity of Suspended Graphene
We show that the optical transparency of suspended graphene is defined by the
fine structure constant, alpha, the parameter that describes coupling between
light and relativistic electrons and is traditionally associated with quantum
electrodynamics rather than condensed matter physics. Despite being only one
atom thick, graphene is found to absorb a significant (pi times alpha=2.3%)
fraction of incident white light, which is a consequence of graphene's unique
electronic structure. This value translates into universal dynamic conductivity
G =e^2/4h_bar within a few percent accuracy
Making graphene visible
Microfabrication of graphene devices used in many experimental studies
currently relies on the fact that graphene crystallites can be visualized using
optical microscopy if prepared on top of silicon wafers with a certain
thickness of silicon dioxide. We study graphene's visibility and show that it
depends strongly on both thickness of silicon dioxide and light wavelength. We
have found that by using monochromatic illumination, graphene can be isolated
for any silicon dioxide thickness, albeit 300 nm (the current standard) and,
especially, approx. 100 nm are most suitable for its visual detection. By using
a Fresnel-law-based model, we quantitatively describe the experimental data
without any fitting parameters.Comment: Since v1: minor changes to text and figures to improve clarity;
references added. Submitted to Applied Physics Letters, 30-Apr-07. 3 pages, 3
figure
Effect of Holstein phonons on the optical conductivity of gapped graphene
We study the optical conductivity of a doped graphene when a sublattice
symmetry breaking is occurred in the presence of the electron-phonon
interaction. Our study is based on the Kubo formula that is established upon
the retarded self-energy. We report new features of both the real and imaginary
parts of the quasiparticle self-energy in the presence of a gap opening. We
find an analytical expression for the renormalized Fermi velocity of massive
Dirac Fermions over broad ranges of electron densities, gap values and the
electron-phonon coupling constants. Finally we conclude that the inclusion of
the renormalized Fermi energy and the band gap effects are indeed crucial to
get reasonable feature for the optical conductivity.Comment: 12 pages, 4 figures. To appear in Eur. Phys. J.
Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene
The non-interacting energy spectrum of graphene and its bilayer counterpart
consists of multiple degeneracies owing to the inherent spin, valley and layer
symmetries. Interactions among charge carriers are expected to spontaneously
break these symmetries, leading to gapped ordered states. In the quantum Hall
regime these states are predicted to be ferromagnetic in nature whereby the
system becomes spin polarized, layer polarized or both. In bilayer graphene,
due to its parabolic dispersion, interaction-induced symmetry breaking is
already expected at zero magnetic field. In this work, the underlying order of
the various broken-symmetry states is investigated in bilayer graphene that is
suspended between top and bottom gate electrodes. By controllably breaking the
spin and sublattice symmetries we are able to deduce the order parameter of the
various quantum Hall ferromagnetic states. At small carrier densities, we
identify for the first time three distinct broken symmetry states, one of which
is consistent with either spontaneously broken time-reversal symmetry or
spontaneously broken rotational symmetry
Schwinger pair creation in multilayer graphene
The low energy effective field model for the multilayer graphene (at ABC
stacking) in external Electric field is considered. The Schwinger pair creation
rate and the vacuum persistence probability are calculated using the semi -
classical approach.Comment: Latex, 5 pages, accepted for publication in JETP let
Nonmagnetic-Defect-Induced Magnetism in Graphene
It is shown that a strong impurity potential induces short-range
antiferromagnetic (ferrimagnetic) order around itself in a Hubbard model on a
half-filled honeycomb lattice. This implies that short-range magnetic order is
induced in monolayer graphene by a nonmagnetic defect such as a vacancy with
full hydrogen termination or a chemisorption defect.Comment: 5 pages, 8 figure
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